1. Field of the Invention
The instant disclosure relates to an inverter; in particular, to a single-phase three-wire inverter.
2. Description of Related Art
Over the past decade the world is facing shortages of many natural resources, and the earth's greenhouse effect is increasingly serious. In order to solve these problems, green energy (e.g. solar energy) is gradually taken seriously. In the beginning, the green energy can reduce the burden of the power plants in rush hour. Further, the green energy source may replace the conventional power plants in order to reduce the production of the greenhouse gases. The technology of solar energy is developing in full swing. Solar panels are usually installed on the roof of the building or house without shelter, and the generated electric power can be integrated to the city power grid or stored in batteries. However, life-time of the battery is limited, and cost of utilizing battery is relatively expensive. Therefore, utilizing an inverter to integrate the electric power of solar energy to the power grid of the power company can reduce the power consumption of power transmission and reduce the power loss, which can improve the efficiency of the power generating system. Additionally, bi-directional function can be added to the inverter for integrating with DC power generating system, in which the generated electric power of the solar energy can be directly provided to DC electronic loads. Therefore, the electric power of the solar energy does not need to be integrated to the city power grid before being converted to DC power and waste of energy can be further reduced accordingly.
At present, many literatures disclose the related topology of the inverter, the controller and the practical functions, but the variation of the inductance (e.g. the inductance variation may be several times of the initial inductance) is still not considered. Due to the material of the iron core, the inductance would vary according to the current of the inductor, such that the current oscillation and the current ripple would be increased, and the precision of the current tracking would be affected thereto especially on the condition of large power, referring to FIG. 1 showing a curve diagram of inductance variation versus current. As shown in FIG. 1, for the system operating in larger power, the inductor can decrease as the current increases. If the controller does not take the variation of the inductance into account, the controller would need to introduce an excessively large compensation to overcome the insufficient inductance, thus there would be a risk of divergence for the system. Therefore, taking the variation of the inductance into account is indispensable for deriving the control law. Further, for the conventional controller of the inverter, the noise generated by switching of the switches often interferes sampling of the feedback signal, thus the controller may oscillate or get wrong functions accordingly. Although, an analog filter can be utilized to filter high frequency noise, the feedback signals would delay and response of the system would be slow, and it would result in distortion of the AC output of the inverter accordingly. Therefore, in recent years, industry and academia workers applies the digital single-chip to perform pulse width modulation control which samples multiple feedback signals in one duty cycle and averages these feedback signals to reduce the influence of high frequency noise. However, sampling multiple feedback signals may not make the feedback current match the reference current in practical, but the processing time of the single-chip is surely increased instead.